Water is made of molecules, each consisting of two hydrogen atoms joined to a single oxygen atom, the composition captured in the familiar formula H2O. This is not a guess or a convention but a fact established beyond any doubt, confirmed by chemistry in countless ways. The simple structure of the water molecule also explains the remarkable properties that make water essential to life.
That water is a compound of hydrogen and oxygen in a fixed ratio of two to one is one of the surest facts in chemistry. It has been demonstrated repeatedly and in many independent ways since the late eighteenth century, when chemists first showed that water is not an element, as the ancients believed, but a combination of two gases. Every measurement since has confirmed the same simple formula.
The composition of water can be proven directly by taking it apart and putting it back together. Passing an electric current through water, a process called electrolysis, splits it into hydrogen and oxygen gas, and the hydrogen is produced in exactly twice the volume of the oxygen, matching the two-to-one ratio of the formula. Conversely, burning hydrogen in oxygen produces water and nothing else. The weights of the elements involved confirm the result precisely.
The water molecule is not straight but bent, with the two hydrogen atoms set at an angle to the oxygen. Because oxygen pulls on the shared electrons more strongly than hydrogen, one side of the molecule carries a slight negative charge and the other a slight positive charge. This makes water a polar molecule, and it is the key to its behaviour, allowing water molecules to cling to one another and to dissolve a vast range of substances.
The polarity and clinging of water molecules explain its many unusual properties. Water can absorb a great deal of heat without changing temperature much, moderating the climate and the inside of living things. Unusually, it expands when it freezes, so ice floats, allowing life to survive beneath frozen lakes. And it dissolves so many substances that it is the medium in which the chemistry of life takes place. All of this flows from the simple molecule H2O.
The fact that water is H2O, and the consequences of that simple structure, underpin chemistry, biology, and the habitability of the planet. Few facts are at once so elementary, so thoroughly proven, and so profound in their reach.
